1. Field of the Invention
The present invention relates generally to high resolution hyperspectral imaging and, more particularly, to space borne high resolution hyperspectral imaging for analyzing atmospheric constituents.
2. Discussion of the Related Art
There is a need in the art for sensing systems to determine the existence of certain atmospheric constituents present in the air. For example, awareness of the potential for rapid global environmental changes has led to a greater interest in the study of the global carbon cycle as it relates to the greenhouse gases CO2 and CH4 in the atmosphere. Space based sensing devices provide the ability to study changes in the concentration levels of these and other atmospheric gases from a remote location.
Known sensing systems have employed different types of sensors, including multi-spectral sensors, hyperspectral sensors, etc., for determining terrestrial, oceanic, and atmospheric properties. Typically, these systems employ sensors that receive reflected and emitted radiation from a field of view and direct the radiation into a spectrograph for analyzing the absorption characteristics of the scene. Hyperspectral imaging is a passive technique that combines spectral resolution with spatial resolution in two dimensions (e.g., the slit and temporal scan dimensions). The hyperspectral sensor creates a large number of spectra at typically low to moderate resolution from contiguous regions of the scene. A dispersing element in a spectrograph associated with the hyperspectral sensor breaks-up the light into its component wavelengths to provide the desired spectral resolution. In order to study atmospheric constituents with high accuracy, high spectral resolution is required. However, for a fixed number of detector channels, spectral coverage decreases as resolving power increases, and specific wavelength regions need to be targeted where certain features of interest are found.
Conventional hyperspectral sensors designed to address this problem have employed multiple sets of telescope plus spectrograph optical configurations for simultaneously measuring a plurality of atmospheric constituents over separate wavelength regions. However, multiple and duplicate optical instruments for studying more than one wavelength region result in a significantly larger system having large volumetric dimensions, mass, etc. For a space based system, it is desirable to deliver optimal payload performance with minimum volume, mass, and other mission-critical payload characteristics.
High spectral resolution also decreases the signal to noise quality of the detected spectrum. To compensate, it is generally necessary to increase the size of the light admitting optics, such as a receiving telescope, to increase the signal to noise quality. However, this also increases the payload characteristics in space based systems. While increases in signal quality are desirable, increases in payload volume and mass characteristics need to be minimized. Hence, there exists a need for an improved high spectral resolution hyperspectral imaging instrument having reduced mission critical payload characteristics for studying multiple atmospheric gases.
In accordance with the teachings of the present invention, a high spectral resolution hyperspectral imaging system arranged within a compact payload volume is disclosed for studying atmospheric components. The system includes a single large aperture optical telescope having an objective lens that receives and focuses light from a particular field of view. A dichroic beam splitter divides the converging light into first (shortwave) and second (longwave) beams. A first spectrograph, including a lens assembly, a diffraction grating and a detector, receives and analyzes the first beam. The first beam propagates through a slit, is collimated by the lens assembly, and strikes the grating in the first spectrograph where it is separated into a first set of predetermined wavelengths. The separated first beam propagates back through the lens assembly acting now as a camera, and is focused onto the detector. A second spectrograph, including a lens assembly, a diffraction grating, and a detector, receives and analyzes the second beam. The second beam propagates through a slit, is collimated by the lens assembly, and strikes the grating in the second spectrograph where it is separated into a second set of predetermined wavelengths. The separated second beam propagates back through the lens assembly acting now as a camera and is focused onto the detector. Thus, a single large aperture telescope is used to receive light for two separate and compact high resolution spectrograph channels to analyze multiple regions of spectrum at high resolving power.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.